The following studies will be performed to elucidate the mechanism by which expression of the early genes of simian virus 40 (SV40) produce oncogenic transformation of mammalian cells: Balb/c 3T3 mouse cells which have been transformed by tsA mutants of SV40 and which are temperature sensitive for maintenance of transformation will be analyzed to determine the role of large T antigen in producing alterations in the frequency of initiation and the temporal sequence of eukaryotic DNA replication. 2. Studies designed to generate mutants which are conditionally defective in small t protein function will be pursued to facilitate the analysis of the role of small t protein in transformation. 3. The regulation of SV40 early gene expression in the transformed state will be investigated by assessing the pattern of integration of SV40 DNA and the presence of various species of SV40 early proteins in transformed cells.